1. Field
The present invention is directed in general to communications systems and methods for operating same. In one aspect, the present invention relates to devices and methods for aperiodic SRS subframe configuration and signaling.
2. Description of the Related Art
In known wireless telecommunications systems, transmission equipment in a base station or access device transmits signals throughout a geographical region known as a cell. As technology has evolved, more advanced equipment has been introduced that can provide services that were not possible previously. This advanced equipment might include, for example, an E-UTRAN (evolved universal terrestrial radio access network) node B (eNB), a base station or other systems and devices. Such advanced or next generation equipment is often referred to as long-term evolution (LTE) equipment, and a packet-based network that uses such equipment is often referred to as an evolved packet system (EPS). An access device is any component, such as a traditional base station or an LTE eNB (Evolved Node B) that can provide user equipment (UE) with access to other components in a telecommunications system.
In mobile communication systems such as an E-UTRAN, the access device provides radio accesses to one or more UEs. The access device comprises a packet scheduler for allocating uplink (UL) and downlink (DL) data transmission resources among all the UEs communicating to the access device. The functions of the scheduler include, among others, dividing the available air interface capacity between the UEs, deciding the resources (e.g. sub-carrier frequencies and timing) to be used for each UE's packet data transmission, and monitoring packet allocation and system load. The scheduler allocates physical layer resources for physical downlink shared channel (PDSCH) and physical uplink shared channel (PUSCH) data transmissions, and sends scheduling information to the UEs through a control channel. The UEs refer to the scheduling information for the timing, frequency, data block size, modulation and coding of uplink and downlink transmissions.
In certain communication standards, such as the 3GPP (3rd Generation Partnership Project) communication standard, uplink spatial multiplexing of up to four layers may be supported by LTE-Advanced. Prior to supporting spatial multiplexing, only a single-antenna port mode of operation was available for the uplink. Thus, the methodology defined in earlier releases of the 3GPP communication standard (e.g., 3GPP Releases 8 and 9) for obtaining channel state information was designed to only measure the channel between a single uplink transmission antenna and the eNB within any single subframe. To support the new uplink MIMO capabilities, it is desirable for the next release of the 3GPP communication standard (e.g., 3GPP release 10) to allow simultaneous channel sounding from multiple UE transmission antennas. Because each uplink transmission antenna requires its own set of orthogonal sounding resources, a new more-efficient sounding methodology is desirable for this next release.
The method used for sounding the channel for the earlier release UEs was known as periodic sounding since this method configures each Radio Resource Control (RRC) Connected UE to transmit a known signal at periodic intervals so that the eNB can measure the channel. Consequently, each UE consumes a fixed amount of resources for that transmission periodically (e.g., every 10 ms) regardless of whether the UE has uplink data to convey or not. To improve the efficiency in the next release, a new aperiodic sounding methodology (i.e., a sounding methodology of irregular occurrence) is being defined which allows the eNB to command the UE to perform aperiodic sounding only when it is required by the eNB. This aperiodic sounding methodology will likely improve efficiency since it will allow the resources to be consumed only when it is beneficial to do so (e.g., only when the UE has uplink data to convey). The new aperiodic sounding methodology is being defined as a complementary mechanism for 3GPP Release 10 and later UEs. The methodology can be used in conjunction with the legacy periodic sounding mechanism in a process where the periodic sounding is configured for each Release 10 RRC Connected UE, but with a longer period (e.g., 20-40 ms or longer) to provide the eNB some information regarding the channel to maintain timing alignment, adjust the UE power control, etc, and then the aperiodic sounding methodology is used to obtain more frequent channel state updates as needed once data comes into the uplink buffer.
In a LTE Release-8 system, the eNB may configure the periodic sounding methodology for a UE to transmit SRS in just one subframe or periodically in multiple subframes. One purpose of a Release 8/9 sounding reference signal (SRS) transmission is to help the eNB estimate the uplink channel quality to support frequency-selective uplink scheduling. In addition, SRS may also be used to control uplink power or uplink timing advance.
Channel sounding is a method used in wireless communication systems to obtain uplink channel state information (CSI) for assigning modulation and coding schemes (MCS), selecting rank and antenna precoding matrix in case of multiple Input and Multiple Output (MIMO) operation, and for frequency selective scheduling for uplink transmission. A known sounding signal waveform is typically transmitted between a transmitter and a receiver, and the channel state information is estimated at the receiver based on the known sounding signal. In 3GPP LTE Release 8, a sounding reference signal (SRS) is typically transmitted periodically from each RRC_CONNECTED UE to the eNB to facilitate uplink timing correction, scheduling and link adaptation. The last symbol of a subframe configured for SRS transmission is used for SRS transmission in LTE Frequency Division Duplexing (FDD) systems as shown in FIG. 1. In LTE, uplink transmissions are organized into radio frames each include 10 subframes ranging from subframe 0 to subframe 9. A subframe is further divided into two slots. In addition, radio frames are indexed from 0 to 1023 and each of the indexed radio frame is referred to as a system frame.
In 3GPP Release 8, cell-specific SRS resources are defined in both frequency and time domains in terms of SRS period, subframe offsets, and SRS bandwidth and are semi-statically configured through RRC signaling in a cell. The cell specific subframe configuration is shown in FIG. 2, and the sounding reference signal subframes are the subframes satisfying └ns/2┌modTSFCεΔSFC, where ns=0, 1, . . . , 19 is the slot index within a frame. For example, the cell-specific SRS subframes when srs-SubframeConfig=0 are the subframes {0,1, 2, 3, 4, 5, 6, 7, 8, 9}, (i.e. all the subframes in each radio frame). In another example, the cell-specific SRS subframes when srs-SubframeConfig=2 are the subframes {1, 3, 5, 7, 9}.
In 3GPP Release 8, SRS bandwidth configuration is shown in FIG. 2 for a system bandwidth between 40 and 60 Resource Blocks (RBs), where one RB includes 12 subcarriers. For a given cell specific SRS bandwidth configuration index CSRS, the absolute SRS bandwidth for a UE-specific bandwidth configuration BSRS depends on the system bandwidth. SRS bandwidth configurations for other system bandwidths can be found in the 3GPP specification. Each UE is assigned semi-statically with a UE-specific periodic SRS resource determined by a UE-specific SRS bandwidth, BSRS, a frequency domain position, nRRC, a transmission comb, kTC, a cyclic shift (CS), a subframe period, TSRS, and a subframe offset, Toffset. In Rel-10 multiple configurations are also supported for UE-specific aperiodic SRS transmissions, which can be triggered by a uplink data grant.
In 3GPP Release 8, UE-specific periodic SRS subframe configuration (subframe period and offset) in LTE FDD is shown in FIG. 3, where the SRS subframes for a UE are the subframes satisfying (10·nf+kSRS−Toffset)modTSRS=0, where nf is the system frame number, and kSRS={0, 1, . . . , 9} is the subframe index within the frame.
In 3GPP Release 10, up to four UE Tx antennas are supported for uplink MIMO. Since separate SRS resources are needed for each Tx antenna, there can be a shortage of SRS resources. To address this issue, aperiodic SRS (A-SRS) has been introduced in which SRS can be dynamically scheduled or triggered by an eNB in an as needed basis through uplink data grants. It has been agreed that similar to the periodic SRS case, each UE is also assigned with a UE-specific A-SRS subframe configuration, i.e. subframes in which A-SRS can be scheduled or triggered. Accordingly, it would be desirable to provide a configuration of UE-specific A-SRS subframes.
One possibility would be to adopt the Release 8 UE-specific periodic SRS subframe configuration as shown in FIG. 4. An issue with this type of configuration is that the configuration limits the A-SRS subframes for a UE to periodical subframes with possible periodicities of 2 ms, 5 ms, 10 ms and so on. This reduces the available opportunities for A-SRS transmission. For example, when the cell-specific SRS subframe configuration of 13 or 14 is used as shown in FIG. 2, the minimum period for UE-specific A-SRS subframes is 5 ms for the odd subframes even though there are 7 or 8 subframes available for SRS transmission within a radio frame. Thus it limits two adjacent A-SRS transmissions to at least 5 ms for UEs configured on those odd subframes.